Mössbauer studies of two-electron centers with negative correlation energy in crystalline and amorphous semiconductors

The results of the study of donor $U^-$-centers of tin and germanium in lead chalcogenides by Mössbauer emission spectroscopy are discussed. The published data regarding the identification of amphoteric $U^-$-centers of tin in glassy binary arsenic and germanium chalcogenides using Mössbauer emission spectroscopy, and in multicomponent chalcogenide glasses using Mössbauer absorption spectroscopy are considered. Published data concerning the identification of two-atom $U^-$-centers of copper in lattices of semimetal copper oxides by Mössbauer emission spectroscopy are analyzed. The published data on the detection of spatial inhomogeneity of the Bose–Einstein condensate in superconducting semiconductors and semimetal compounds, and on the existence of the correlation between the electron density in lattice sites and the superconducting transition temperature are presented. The principal possibility of using Mössbauer $U^-$-centers as a tool for studying the Bose–Einstein condensation of electron pairs during the superconducting phase transition in semiconductors and semimetals is considered.